The present invention relates to a system and method for monitoring the proper operational condition of a Bowden cable, and more particularly to detecting friction-induced wear within a Bowden cable used in a system for clamping a tube containing medical fluids, thereby enabling maintenance or replacement of the cable prior to failure.
Many medical procedures involve transporting fluids such as blood or medicinal solutions within flexible tubing. Is it often necessary for the safety and efficacy of the medical procedure to control the flow rate of the fluids being transported. One common means of controlling the flow rate is to occlude the tubing by applying a compressive clamping force to it, thereby constricting the passage of the fluid within the tube.
Various systems have been used to control the flow rate of medical fluids by applying a compressive clamping force to flexible tubing. Typical clamping systems have used a solenoid-driven piston to pinch the tubing against a plate. Such systems have been relatively heavy, resulting in the need for mounting supports for the clamp.
A newly improved clamping system including a flexible cable assembly component commonly referred to as a Bowden cable for transmitting a mechanical clamping force from a remote driver to an actuator which is in contact with the tubing is described in copending application U.S. serial number (17916), entitled “Light Weight Tube Clamp for Medical Fluids,” filed contemporaneously herewith. This application is hereby incorporated by reference in its entirety.
Bowden cables comprise a flexible hollow outer casing containing a slidable inner wire or wire rope. Bowden cables are well known for their use in conjunction with bicycle brakes. By mechanically anchoring the outer casing and permitting the inner wire to slide within the outer casing, the Bowden cable can transmit longitudinal pulling or pushing forces from one end of the inner wire to the other.
The cylindrical outer casing of a Bowden cable includes a coiled metal wire or rod to provide longitudinal stiffness together with the ability to easily bend the Bowden cable along a desired path. To minimize the amount of force required to operate the Bowden cable, it is desirable to minimize the friction acting upon the inner wire as it slides within the casing. Since the inner wire is also usually made of metal, the resistance would be relatively high if it directly contacted the coiled metal wire of the casing. To that end, Bowden cables typically include a friction-reducing material layer disposed between the outer casing and inner wire, usually in the form of a cylindrical plastic liner.
Over long periods of use, the friction-reducing material layer in a Bowden cable is prone to eventually wear out. It can also be damaged through kinking of the Bowden cable. Any break in the layer can result in loss of its ability to reduce the friction. When that happens, the driving system (such as a motor) of the Bowden cable may be inadequate to function effectively, and the system in which it is installed may become inoperable.
A malfunction of a Bowden cable employed in a medical device, such as a tube clamp system for medical fluids, is particularly critical. Therefore, a means for monitoring the physical integrity and performance of the friction reducing material layer in a Bowden cable used in a tube clamp system for medical fluids, thereby enabling maintenance or replacement of the cable prior to failure, is highly desirable.